advice on building a simple event timer with simple i/o and simple calculation

[tanlux]

Hello everybody,

I'm trying to design a device that can time 2 sensor inputs (I want the time interval between inputs, accurate to 1uS). I also want the user to be able to input 2 numbers. I then want to perform a simple calculation using the user inputs, 1 variable (temperature from another sensor) and the time interval. I then want to output a floating point/integer number to the user.

I've built a prototype using an Arduino (to prove the concept) but I want to be able to mass produce the device and therefore build a cheap product. Using an Arduino is like using a tank to kill an ant.
Can anyone suggest the hardware I should be using to build such a device?

Thanks.

 

[Rixen]

Sounds like you already developed the thing.

Dont know which Arduino you used, but the Atmega328 is fairly inexpensive, did you consider just embedding it on it's own?

 

[tanlux]

Arduino Uno. Surely there's a simpler way to get a user i/o interface (via lcd) and doing timing and the calculation? I've been out of digital electronics for a while and sort of lost track of what's currently available.

 

[Doug3004]

The Nano is 5/8 x 1-5/8 inches; listings on Aliexpress show them as 10 for ~$20. It has a mini-B style USB plug so it's easy to program in the Arduino IDE. You just need have the correct cable.

The Arduino pro mini is a bit smaller and has no USB interface. China sellers are listing clones for 10 for ~$13. This is probably the easiest way to go (since you already have Arduino code) and it looks the cheapest too. Note that there is 3.3v-pin versions and 5v-pin versions of the Pro Mini, and 8MHz versions and 16MHz versions as well.

Neither really does true microsecond timing IIRC. The micros() call on the 16Mhz "Arduino" chips are only accurate to divisions of 4 microseconds.

You can do low-level programming to get better than that:
http://www.instructables.com/id/How-to-get-an-Arduino-micros-function-with-05us-pr/

...Or you can use some other board with a better clock function.

There is people selling 10pcs/lot STM32F103C8T6 ARM STM32 boards for ~$1.80 each.
http://www.st.com/en/microcontrollers/stm32f103c8.html
It is about the same size as the Nano but is 3.3v and has no USB chip onboard...
This page tells how to set up the Arduino IDE, or you can use another environment/toolchain entirely.
http://grauonline.de/wordpress/?page_id=1004

Apparently it can do 1uS timing but as you need to use Assembly-
http://www.carminenoviello.com/2015/09/04/precisely-measure-microseconds-stm32/
AVR Studio allows Assembly and supports the Atmel chip boards, but it may not support the STM32 boards.
This page-
https://www.cs.nmsu.edu/~jcook/arduino/index.php?n=Notes.AssemblyMods
shows how to get the Arduino IDE to accept Assembly source files.

Did you really need microsecond timing?
If it's working on an Arduino now and you're not using a low-level timing method, you can save a lot of bother by skipping that detail.

Other possible small boards may the digispark pro, but even the China clones of it are not as cheap as the ones above. I think Chin wants like $4-$5 for them.

The ESP8266 boards are cheap, but you'd need the later ones to get many I/O pins and they cost $2 - $3 each. A later one would be the ESP-12E or F.... If you bought a few dozen at once you may get a price near the ARM STM32 boards mentioned earlier.

 

[tanlux]

Thanks for that Doug. I've been revisiting my calculations and actually 100uS is tolerable for now. I was thinking ahead too far to possible further uses. I think a smaller cheaper 'off the shelf' board is the best way to go, rather than something custom built. The nano looks like the best candidate.

 

[Doug3004]

Thanks for that Doug. I've been revisiting my calculations and actually 100uS is tolerable for now. I was thinking ahead too far to possible further uses. I think a smaller cheaper 'off the shelf' board is the best way to go, rather than something custom built. The nano looks like the best candidate.

The Pro Mini clone is even cheaper and smaller tho... You just need to get a serial-to-USB adapter (or two) to program it.

The cheapest serial-to-USB adapters cost like $2 from China, and there's about four different chips that are common. They only support USB 1.0, but that's not a major drawback for this use. They usually all work from 9600 baud up to at least 115200 baud but the Chinese ones claiming to have FTDI chips are usually counterfeit; they only work with one old FTDI driver. I don't recommend buying those.
,,,,,,
One of these chips (not the fake FTDI one) is odd in that it takes Windows 10 up to 5 minutes or more to recognize it and install the drivers--but it does work just fine after that. If you connect it for the first time and you don't hear Windows make its "new USB device" song and dance, just leave it connected for a while. Eventually it gets it.

You're not in the same country but anyway:
Adafruit has one for $6 that probably works right; Sparkfun sells an adapter with a genuine FTDI chip for $15.
The Sparkfun/FTDI adapter is probably the best option to have (least errors, fastest/USB 2.0 and best driver support) but it's not necessary. Many people only use the cheap Chinese ones.